Flexography is a method of printing whereby a flexible plate with a relief image is situated around a cylinder and its relief image is inked up and the ink then transferred to a suitable substrate. The process has mainly been used in the packaging industry where the plates could be sufficiently soft and the contact sufficiently gentle to print on uneven substrates such as corrugated cardboard as well as flexible materials such as polypropylene film. The quality of the printing was far inferior to processes such as lithography and gravure, but nevertheless found markets that were applicable to the process. In order to accommodate the various types of substrates, flexographic plates have to have a rubbery or elastomeric nature whose precise properties must be adjusted for each particular substrate.
The methods of producing flexographic plates have shown progress over the years, although the various historical means of producing plates still find places in the present market. Initially, flexographic plates were made by cutting the relief image into a sheet of rubber with a knife. An improvement was achieved by forming a mold that could be produced by photo-etched graphics and then pouring rubber into the mold and vulcanizing to form the plate. This produced much finer and more accurate images, and it started to be worthwhile to compensate for image distortion when the plate was bent around the printing cylinder. A further improvement came about with the development of liquid photopolymers. Mixtures of such materials could be poured into a framework and exposed by UV light from the back to produce the floor of the plate and exposed by ultraviolet light from the front through a negative working photo tool to produce the relief image. The UV light hardened the material in the image areas and the un-imaged liquid photopolymer could be washed away with solvent. Such a process is instanced in U.S. Pat. No. 3,794,494 by Kal et als. and in U.S. Pat. No. 3,960,572 by Ibata et al. The liquid mixture of polymers was sold to the customer to prepare and expose the plates. These mixtures are referred to in this application as liquid photopolymers and the plates made by this process are referred to as liquid photopolymer plates albeit that after exposure and development they are completely solid.
U.S. Pat. No. 4,323,636 by Chen describes the use of thermoplastic elastomeric block copolymers. Instances of these block copolymers are those made by Shell Chemical Company and sold under the trademark of “Kraton”. They are used in conjunction with an acrylate or methacrylate monomer and a photoinitiator. The layer can be formed by solvent deposit or extrusion and the plate material can be bonded to a base substrate. The upper surface may have on it a thin hard flexible solvent soluble coating and on top of this a strippable thin film of e.g. polyethylene to protect the plate during storage. This would then constitute a flexographic printing blank that can be sold to the customer as a solid plate, imaged by ultra-violet exposure through a negative mask, and the un-polymerized material washed away with solvent. The term “blank” is used in this application to describe unimaged plates. Such plates are usually of a thickness of one or more millimeters. In this application they will be referred to as solid plates as the blanks are solid. Solid plates are regarded as an improvement over the liquid photopolymer plates because they are easier to handle and prepare for imaging.
U.S. Pat. No. 4,994,344 by Kurtz et al. is entitled Sheetlike Light-Sensitive Recording and describes flexographic printing blanks made using ethylene-propylene-alkadiene terpolymers with a photopolymeric initiator, monomer and inhibitor of thermally initiated polymerization. It includes the process of initial back exposure to establish the floor of the plate before image exposure from the front of the plate through a negative mask.
U.S. Pat. No. 5,719,009 by Fan describes an invention that typifies the next significant development in flexographic plate processing. This invention eliminates the need for a negative phototool because it is integral in the flexo plate itself. The flexoplate comprises solid photosensitive layers as described in previously mentioned patents. The plates of this invention have an over-layer containing carbon black with a binder resin. The black layer is ablated with an infrared laser in response to a digital signal received in response to a pattern shown on a computer. Digital imaging using a modulated laser source is an important part of the general technology that has become known as computer-to-plate (CTP) and is used for instance in the production of offset lithographic printing plates. In the case of the Fan '009 invention, the energy used to ablate the integral photo-tool has to be significantly higher than that in imaging CTP litho plates and energies up to 3.6 joules per square centimeter are mentioned in the Fan '009 patent. The ablated areas in the carbon coating permit UV light to expose the sensitive elastomeric layer and harden it. The other unexposed areas situated under the unablated carbon layer are washed away together with the remains of the carbon layer, leaving a relief image.
Although the Fan invention shows significant improvement in image quality and ease of handling of the plate, as well as process simplification, it has long been recognized that the simplest way of making a flexographic printing plate would be by direct engraving with a laser beam, eliminating all need for washing or drying the plate or multiple types of exposure.
Caddell in U.S. Pat. No. 3,549,733 describes the formation of a laser engraved relief printing plate. The preferred radiation source is a CO2 laser and preferred polymers were those that did not form ridges around the image areas. The plates described would not have the elastomeric properties needed for flexographic printing but could be used in letterpress printing. Letterpress printing differs significantly from flexographic printing in that it is more akin to lithography in the complexity of the printing machine and the type of ink used. Letterpress inks must be high viscosity paste-like, similar to offset inks and do not in general contain very volatile solvents. If the letterpress printing is via an offset blanket the printing process is termed dry offset. As with offset printing, dry offset and letterpress require high pressure between the plate and blanket or substrate to achieve good ink transfer, whereas flexographic printing uses the minimum pressure possible. Thus a letterpress plate would be unsuitable for flexo printing as it would not give good ink transfer under low pressure and similarly a flexographic plate would be unsuitable for letterpresses as the high pressure would distort the softer plate and give very poor image quality with huge dot gain.
U.S. Pat. No. 5,259,311 by McCaughey Jr. directly relates to laser engraving of flexographic polymeric printing plates. However, this process, whilst employing a carbon dioxide laser for imaging, needs several other UV flood exposure steps and washing out of imaged material.
Cushner et als. in U.S. Pat. Nos. 5,798,202 and 5,804,353 describe single or multiple layers of elastomers for direct laser engraving of flexographic plates with various methods or combination of methods of reinforcement of the layers, including where any chemical reinforcement of the layers is inter als not a peroxide. The patents use (although not the preferred IR absorbers) dyes etc. and may involve additional UV curing stages. Imaging sensitivity is limited by the use of large quantities of block polymers such as the Kratons. Poor melt edges are reported for flexographic engraving of mixtures containing such polymers by Hiller in US Published Application 2002/01369A1 in Comparative Examples.
Gelbart in U.S. Pat. Nos. 6,090,529 and 6,159,659 claims elastomeric foams with a sealing top layer of the same chemical nature as the foam material, for laser engraving to produce flexographic plates. Such material can be more easily ablated or collapsed during laser engraving as the density of the plate material is reduced by the foam cells. The foam may include microspheres with either glass or plastic walls.
Hiller at als. In U.S. Pat. No. 6,511,784 and in US 2002/0136969A1 claim laser engraving of flexographic printing plates comprising silicone rubbers and laser absorbing fillers such as iron oxide or carbon black
US2003/0129530 by Leinenbach et als. claims a method for laser engravable flexographic printing elements on flexible metallic supports. The actual engravable layer contains an elastomeric binder, an absorber of radiation, an evaporatable solvent and a polymerization initiator.
US2003/0136285 by Telser et als. describes a method of preparing flexo plates for laser engraving in which the plate is first cross-linked on the surface by UV or heat. This patent, as with previous patents, employs mixtures dissolved in solvent and deposited from the solvent. The disadvantage of the use of solvent is that it has to be thoroughly removed during plate manufacture. If the mixture is deposited by coating methods, it has to be done in several passes because the thickness of the plate demands this approach. Otherwise, the solvent escapes from the coating as bubbles as it dries on the surface before the solvent escapes from the bulk of the material. If molding is attempted, the mold shows shrinkage during solidification.
US2003/0180636 by Kanga et al describes laser engraving flexo plates where a UV sensitive material is mixed with hydrocarbon-filled plastic microspheres and an infra red (IR) dye. The composition is heated to expand the microspheres and extruded to form a plate. The plate is then engraved with an infrared laser. The energy causes the foam to collapse and then the plate is UV cured to harden it off. This patent addresses the problem also expressed in the Gelbart patents ('529 and '659) that laser engraving is long and tedious—especially with low-powered laser diodes—and that carbon dioxide lasers lack beam resolution and cause anomalies due to heat dissipation.
Despite the limitations of CO2 lasers they are now being used commercially in flexo engraving machines. They have a reputation for slow and expensive imaging of limited resolution related to the 10.6 micron wavelength of the imaging radiation produced by the CO2 laser. The attractions of direct engraving are sufficient to ensure commercial use where fast imaging and high print quality are not required. However, it would be preferable to use infrared diodes which produce radiation in the near infrared—approximately 800 to 1100 nm—and have the advantages of high resolution and relatively low laser cost so that they can be used in large arrays. Up until now, although the use of such lasers is claimed in many engraving patents, they have not been of industrial application because even combined with the most sensitive plates available, satisfactory engraving could not be achieved.
With the recent availability of IR laser diodes that can produce power of 8 watts or greater—herein referred to as high power lasers diodes—the inventor has now found it possible to formulate flexographic printing plates which can be rapidly engraved using such lasers. Moreover, such plates have distinct advantages over the prior art in ease of production, ease of use and resolution of image.